Investigations of the dynamical behaviors of a millimeter-scale cavitation bubble near the rigid wall

The collapse of the cavitation bubble near the rigid wall emits shock waves and creates micro-jet,causing cavitation damage and operation instability of the hydraulic machinery.In this paper,the millimeter-scale bubble near the rigid wall was investigated experimentally and numerically with the help of a laser photogrammetry system with nanosecond-micron space-time resolution and the open source package OpenFOAM-2212.The morphological characteristics of the bubble during its growth phase,collapse phase and rebound phase were observed by experiment and numerical simulation,and characteristics of the accompanying phenomena including the shock wave propagation and micro-jet evolution were well elucidated.The numerical results agree well with the experimental data.The bubble starts from a tiny small size with high internal pressure and expands into a sphere with a radius of 1.07 mm for γ=d/R max =1.78.The bubble collapses into a heart shape and moves towards to the rigid wall during its collapse phase,resulting in a higher pressure load for the rigid wall in the second collapse.The maximum pressure of the shock wave of the first bubble collapse phase reaches 5.4 MPa,and the velocity of the micro-jet reaches approximately 100 m/s.This study enriches the existing experimental and numerical results of the dynamics of the near-wall cavitation bubble.     

Experimental study of the dynamics of a single cavitation bubble excited by a focused laser near the boundary of a rigid wall

Based on high-speed photographic experiments,this study presents a detailed qualitative and quantitative analysis of the dynamics of a single cavitation bubble near the boundary of a rigid wall in asymmetric settings.The main findings are reported as follows:(1)The non-sphericity of the bubble interface decreases with increasing spacing between the bubble and the boundary,and the asymmetry of the bubble becomes more significant with increasing asymmetry angle.(2)The motion mode of the bubble cluster in the second oscillation cycle can be divided into two typical modes depending on the direction of movement.(3)The angle between the oblique jet pointing towards the upper wall surface and the horizontal direction in the second oscillation cycle decreases as the dimensionless spacing decreases.     

Numerical investigation of the interactions between a laser-generated bubble and a particle near a solid wall

The interactions between the bubbles and the particles near structures are important issues for the applications of the cavitation in the fluid machinery.To stu...     

Investigation of cavitation bubble collapse near rigid boundary by lattice Boltzmann method

The dynamics of the bubble collapse near a rigid boundary is a fundamental issue for the bubble collapse application and prevention. In this paper, the bubble collapse is modeled by adopting the lattice Boltzmann method(LBM) and is verified, and then the dynamic characteristics of the collapsing bubble with the second collapse is investigated. The widely used Shan-Chen model in the LBM multiphase community is modified by coupling with the Carnahan-Starling equation of state(C-S EOS) and the exact difference method(EDM) for the forcing term treatment. The simulation results of the bubble profile evolution by the LBM are in excellent agreements with the theoretical and experimental results. From the two-dimensional pressure field evolution, the dynamic characteristics of the different parts during the bubble collapse stage are studied. The role of the second collapse in the rigid boundary damage is discussed, and the impeding effect between two collapses is demonstrated.     

空泡在固壁附近潰灭的数值模拟

实验表明,静止流场中的近壁空泡在泡内外压差的驱动下将发生变形收缩,直至完全溃灭消失。在这一过程中,伴随着空泡形状的变化,流场中出现多个高压脉冲和高速射流。本文通过直接求解原始变量的N-S方程,用VOF方法计算气液两相交界面的运动过程,对这一现象进行了数值模拟和分析。结果表明理论分析和实验现象相吻合。     

Influence of bubble-particle distance on the dynamic behaviors of a cavitation bubble near a particle

Cavitation and silt-erosion often co-exist,causing severe damage on fluid machinery.In this paper,the dynamic behavior of a cavitation bubble near a fixed spherical particle is numerically studied,with the focus on the influence of the stand-off distance γ on the bubble collapse morphology,micro-jet velocities and pressure on the particle.With the increase in the value of y,the bubble profile in the collapse stage exhibits three distinct characteristics:Mushroom-shaped,pear-shaped and spherical-shaped,and the corresponding micro-jets are identified as contact jet,non-contact jet,and long-distance jet.All studied distances can be categorized into three ranges,and the typical cases in each range are demonstrated.The maximum jet velocity Vmax and the maximum pressure difference between the upper and the bottom of the particle △pmax show the highest peak at y=0.9,with Vmax up to 180 m/s and△pmax up to 10.8 MPa.     

Experimental study of the interaction between the spark-induced cavitation bubble and the air bubble

Experiments are carried out by using high-speed photography to investigate the interaction between the spark-generated cavitation bubble and the air bubble in its surrounding fluid. Three problems are discussed in detail： the impact of the air bubble upon the development of the cavitation bubble, the evolution of the air bubble under the influence of the cavitation bubble, and the change of the fluid pressure during the development of a micro jet of the cavitation bubble. Based on the experimental results, under the condition of no air bubble present, the lifetime of the cavitation bubble from expansion to contraction increases with the increase of the maximum radius. On the other hand, when there is an air bubble present, different sized cavitation bubbles have similarity with one another generally in terms of the lifetime from expansion to contraction, which does not depend on the maximum radius. Also, with the presence of an air bubble, the lifetime of the smaller cavitation bubble is extended while that of the bigger ones reduced. Furthermore, it is shown in the experiment that the low pressure formed in the opposite direction to the cavitation bubble micro jet makes the air bubble in the low pressure area being stretched into a steplike shape.     

Determination of the shedding frequency of cavitation cloud in a submerged cavitation jet based on high-speed photography images

To accurately determine the shedding frequency of the cavitation cloud in a submerged cavitation jet,the spectral analysis and the proper orthogonal decomposition(POD)for high-speed photography images are performed.The spectrums of 6 different kinds of image signals(the area-averaged gray level,the line-averaged gray level,the point gray level,the cavitation length,width,and area)are calculated and compared.The line-averaged gray level is found to be optimal in determining the shedding frequency but an accurate frequency can only be obtained in the stable-frequency zone where the cavitation cloud sheds.In repeated experiments,the plateau-shape distribution of the main frequency is established with a deviation of 10.8%.A revised Reynolds number Re'is defined and the shedding frequency can be correlated to Re'by a power law when the cavitation number is less than 0.02.This relationship is validated by the experimental data in literature.The first mode of the POD characterizes the ensemble-average of the cavitation cloud while the second mode is the major part of the cavitation cloud transient components.The modes 2-5 are organized in pairs,which confirms the periodic feature of the cavitation cloud in the submerged cavitation jet.Near the nozzle exit,the modes 2-5 are symmetrically distributed in the jet shear layer.The shedding frequency of the cloud cavitation can also be precisely determined by performing the spectral analysis of the weighting coefficients of the mode 2.This paper shows that the two parameters,namely,the line-averaged gray level and the weighting coefficients of the mode 2,can be confidently used to calculate the shedding frequency of the cavitation cloud in a submerged cavitation jet.     

On study of non-spherical bubble collapse near a rigid boundary

This paper applies numerical methods to investigate the non-spherical bubble collapse near a rigid boundary. A three-dimensional model, with the mass conservation equation reformulated for considering the compressibility effect, is built to deal with the coupling between the pressure and the flow velocity in the momentum and energy equations and to simulate the temporal evolution of the single bubble oscillation and its surrounding flow structure. The investigations focus on the global bubble patterns and its schlieren contours, as well as the high-speed jet accompanied when the bubble collapses and the counter jet is generated in the rebound stage. The results show that the robust pressure waves emitted due to the bubble collapse lead to substantial changes of the flow structures around the bubble, especially the formation of the counter jet generated in the rebound stage. Furthermore, compared with the high-speed jet when the bubble collapses, the counter jet in the rebound stage emits the momentum several times greater in the magnitude and in diametrically opposite direction at the monitoring point.     

A thermal-hydrodynamic coupling method for simulating the interplay between bubble departure and wall temperature variation in nucleate boiling

A thermal-hydrodynamic coupling model is used to investigate the interplay between the wall temperature distribution and the bubble departure during the nucleat...     

An experimental investigation of transient cavitation control on a hydrofoil using hemispherical vortex generators

Unsteady cavitation causes noise,damage,and performance decline in the marine engineering and fluid machinery systems.Therefore,finding a method to control the ...     

An experimental study on the impact behavior of cavitation inside tip clearance of a hydrofoil

Tip clearance cavitation is one of the most common cavitation phenomena exist on duct propellers,pumps and some hydraulic turbines,which may lead to erosion of the components.Due to the influence of the nearby wall,cavitation inside the tip clearance is more complicated than other cases without interaction.So far,the understanding about the impact mechanism of tip clearance cavitation is still limited.In this paper,to obtain the impact behavior of tip clearance cavitation,a high-speed camera was used to capture the cavitation behavior inside the tip clearance of a hydrofoil,and surface paint coating peeling method was applied to show the impact region.Results indicated that cavitation around the tip of the hydrofoil was composed of a tip separation cavity and a tip leakage vortex cavity,and the one with contribution to impact was the tip separation cavity.Through the comprehensive analysis of the paint peeling region and dynamic behavior of tip separation cavity,the impact was found to be related to the local collapse and rebound of the cloud cavitation shed from the attached part.In addition,the influence of tip clearance size on the behavior of tip clearance cavitation was also investigated.As the tip clearance size increased,the tip separation cavity tended to transfer from sheet cavitation to vortex cavitation.These findings can provide a sound basis for evaluating the erosion risk arising from the tip clearance cavitation.     

LES investigation of the tip vortex cavitating flow with special emphasis on the interaction between cavitation and vorticity by a modified cavitation model

Cavitation within the tip vortex(TV)flow remains a challenging issue in the design of high-speed and low-noise hydraulic machinery.In this paper,the TV cavitating flow around an elliptical hydrofoil is calculated by using large eddy simulation(LES)combined with a modified Schnerr-Sauer(S-S)cavitation model.The original S-S cavitation model is modified by taking into account the typical effect of vortex flow.The partial pressure term which can describe the vortex quantitatively and qualitatively is confirmed asρmω2xr2c and is considered into the R-P equation of the modified S-S cavitation model.Comparison between the numerical and experimental results shows good agreement in the form and evolution of cavities,including attached cavities(AC)and tip vortex cavities(TVC).The vorticity transport equation is utilized to investigate the dynamic mechanisms of the vortex development around the TVC.Further analyses indicate that cavitation in the TV flow influences the pressure in the core of the cavity and the local flow patterns.Typical vortex structures in the TV cavitating flow include TV,secondary vortex(SV)and wake vortex(WV).The direction and magnitude of the rotation effect can be described by axial vorticity which is drawn on the iso-surface of Q=1×105s-2.The development of the TV cavitating flow can be divided into two stages:Stage I,the development and fusion of TV,SV,stage Ⅱ,the dissipation of SV.The stretching term dominates the evolution of TV,and the dilatation term is the main reason in the mergence process of SV.     

振荡流边界层中颗粒受力的三维格子玻耳兹曼模拟

应用格子玻耳兹曼方法对振荡流边界层中单个颗粒在近壁面处的受力进行了三维模拟,计算结果与已有数值模拟结果(雷诺数为300)和实验结果(雷诺数为2000)进行了比较,吻合均较好。计算结果表明,当雷诺数为300时,在振荡流做正反向运动时力系数的幅值变化过程相同,横向涡量的空间分布具有对称性。雷诺数为2000时,紊动增强使力系数在不同流动方向时的变化有所差异,而且横向涡量的分布呈现非对称性,这时颗粒受到横向力作用。     

Effect of liquid viscoelasticity on the interactions between cavitation bubbles and free surfaces

The influence of liquid viscoelasticity on the interaction between cavitation bubbles and free surfaces is of great practical significance in understanding bubble dynamics in biological systems.A series of millimeter cavitation bubbles were induced by laser near the free surfaces of the water and viscoelastic polyacrylamide(PAM)solutions with different concentrations.The effects of liquid viscoelasticity on the interactions of cavitation bubbles with free surfaces are analyzed from the perspectives of the evolution of free surface and bubble dynamics.The experimental results show that as the dimensionless standoff distance increases,the evolutions of free surface behaviors in all experimental fluids can be divided into six types of water mounds,i.e.,breaking wrinkles,spraying water film,crown,swallowed water spike,hillock,and slight bulge.All the critical values of the dimensionless distance dividing different types decrease with increasing concentration.The evolutions of first four types of water mounds in PAM solutions differ from those in the water.Water droplets splashing in different directions are produced around the breaking wrinkles in the water.Meanwhile,the breaking wrinkles in PAM solution move with the\"liquid filaments\"towards the central axis.The water spike in the pattern of spraying water film in PAM solution is more stable than that in the water.As the solution concentration increases,the water skirt in the pattern of crown contracts earlier and faster,and the rate of increase in the height of the water skirt decreases.For swallowed water spike in PAM solution,the upper part of the newly formed water spike is not significantly thicker than the middle part,and thus the water waist structure does not form.Liquid viscoelasticity inhibits the bubble growth and collapse,and the bubble migration as well,especially in the second period.Shorter and thicker cavities are formed in PAM solutions with higher concentration,while slender and stable cavities formed in the water at the same dimensionless distance.The velocity and displacement of the tip of bullet jet both decrease as the solution concentration increases.     

双孤立波直墙爬高的实验研究

海啸波在近海传播过程中有可能会演化成若干个孤立波组成的波列.为了研究孤立波波列对海岸结构物的作用机理,本文在波浪水槽中进行了双孤立波的直墙爬高实验.通过控制推板式造波机的运动,有效地实现了不同波峰间距双孤立波的造波方法.在复演并验证单孤立波直墙爬高的基础上,完成了不同相对波高、相对波峰间距的双孤立波爬高系列实验,给出了双孤立波平底传播及直墙爬高过程的波面时间序列及爬高过程水线运动时间序列.实验结果表明:对两个等波高的双孤立波而言,当相对波峰间距降到某临界值时,跟随在后孤立波的爬高放大系数将小于先导孤立波的爬高放大系数;当先导孤立波的波高大于跟随孤立波的波高,先导孤立波将更加显著地抑制对跟随孤立波的爬高过程,使得后者的最高爬放大系数显著减小;若在直墙爬高前两个波已明显重叠,即呈双峰瞬态波形,爬高放大系数不会大于具有相同波高的单孤立波的爬高放大系数.     

水平井筒变质量分散泡状流压降的理论与实验研究

由于水平井筒和常规水平管道中气液两相流动的相似和差别,可以预知常规水平管流的压降计算方法对于井筒流动来说就需要进行修正或扩展。本文对气、液两相分别应用质量守恒方程和动量守恒方程,考虑管壁存在人流或出流对于分层流流型压降的影响,得到水平井筒气液两相变质量流动分散泡状流流型的压降计算方法。同时,设计并建立了水平井筒流体流动模拟实验装置,在轴向为气液两相流动的前提下分别进行了上管壁单孔眼注入和下管壁单孔眼注入的压降实验研究,获得了大量的实验数据。实验数据和理论计算结果吻合很好,这表明该计算方法具有实际应用价值。     

Origins of filter effluent particles: experimental study of particle deposition and detachment.

This paper investigates the relative roles of particle deposition and detachment in controlling the origin of filter effluent particles. A conceptual mathematical model was developed and laboratory-scale experiments were conducted. Laboratory experiments were performed using three sizes of fluorescent microspheres (FMs), to determine the fraction of filter effluent particles that are filter influent particles that were never removed, as well as the fraction of filter effluent particles that were detached after deposition. Experimental results indicated that particle detachment is significant beginning from the early phase of filtration. FM removal increased with filter run time, depth and particle size. For each size FM at one filter depth, FM removal increased with filter runtime to a maximum due to ripening and then decreased with filter runtime after ripening due to limited pore space remaining in the filter. The fraction of effluent particles that were detached particles increased with particle size and filter bed depth.